Photographic processes



Oct. 4, 1966 M. GREEN 3,276,873

PHOTOGRAPHIC PROCESSES Filed Oct. 23, 1965 SUPPORT PHOTOSENSITIVE LAYER PROCESSING COMPOSITION IMAGE-RECEIVING LAYER SUPPORT FIG.I

SPREADER SHEET PROCESSING COMPOSITION PHOTOSENSITIVE LAYER IMAGE-RECEIVING LAYER SUPPORT FIG.2

INVENTOR.

MILTON GREEN BROWN and MIKULKA and 7 ROBERT M. FORD ATTORNEYS United States Patent 3,276,873 PHOTOGRAPHIC PROCESSES Milton Green, Newton, Mass., assignor to Polaroid Corporation, Cambridge, Mass., a corporation of Delaware Filed Oct. 23, 1965, Ser. No. 503,675 7 Claims. (Cl. 96-29) This application is a continuation-impart of my copending US. application, Serial No. 305,791, filed August 30, 1963, now abandoned, which is, in turn, a continuation-in-part of US. application, Serial No. 834,262, filed August 17, 1959 (now abandoned).

The present invention relates to photography and more particularly to high speed diffusion processes wherein a silver halide photosensitive emulsion has been under exposed in relation to its A.S.A. Exposure Index, that is, to processes for deriving useful photographic transfer prints from latent images formed at low exposure levels.

In diffusion transfer processes, for the tormation of positive silver images, a. latent image contained in a selectively photoexposed photosensitive silver halide emulsion is developed. Almost concurrently therewith, a soluble silver complex is obtained by reaction of a silver halide solvent with the unexposed and undeveloped silver halide of said emulsion. Preferably, the photosensitive silver halide emulsion is developed with a processing composition in a viscous condition, which is spread between the photosensitive element comprising the silver halide emulsion and a print-receiving element comprising, preferably, a suitable silver precipitating layer. The processing composition effects development of the latent image in the emulsion and substantially contemporaneous therewith forms a soluble silver complex, for example, a thiosulfate or thiocyanate complex, with undeveloped silver halide. This soluble silver complex is, at least in part, transported in the direction of the print-receiving element and the silver thereof is largely precipitated in the silver precipitating layer of said element to form the desired positive image therein.

High speed diffusion transfer processes of the present invention are generally of the type in which, for example, a silver halide stratum containing a latent image formed at a low exposure level and an image-receiving stratuim, in superposition, are subjected to a processing composition containing a highly energetic silver halide developing agent set forth hereinafter and a silver halide solvent in order to form a silver transfer print in and/or on the imagereceiving stratum. The silver halide developing agent serves to reduce photoexposed silver halide to silver in the photosensitive emulsion stratum. The silver halide solvent reacts with unreduced silver halide to form the aforementioned soluble silver complex which, in turn, is reduced in the presence of the image-receiving stratum to form the desired positive print. The photosensitive stratum may be subsequently dissociated from the image-receiving stratum. In accordance with the present invention, the silver halide stratum may be underexposed in relation to its rated A.S.A. Exposure Index and the silver halide developing agent is specifically selected from the class hereinafter set forth.

Preferably, the image-receiving stratum is in such condition as to cause silver reduced there, in comparison with silver reduced in the photosensitive silver halide stratum, to possess very high covering power, that is, opacity per given mass of reduced silver. This high covering power is achieved by accumulating the silver deposited in the silver-receptive stratum in unusually dense masses, for example, by minimizing the thickness of the stratum in which the silver-receptive material is contained.

The high speed diffusion transfer processes set forth herein provide readily available and uniquely simple procice esses for producing satisfactory high quality prints of good resolution and trivial granularity at low illumination levels, for example, at overall exposures that are equivalent to A.S.A. Exposure Indices of 700 and up.

Accordingly, objects of the present invention are: to provide novel high speed diffusion transfer processes wherein a selectively photoexposed silver halide emulsion has been substantially underexposed in relation to its A.S.A. Exposure Index; and to employ, in processes of the aforementioned type, as a silver halide developing agent, the agents designated hereinafter.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the processes involving the several steps and the relation and order of one or more of such steps with respect to each of the others which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

Fora fuller understanding of the nature and objects of the present invention, reference should be had to the following detailed description taken in conjunction with the accompanying drawing wherein:

FIGURE 1 is a diagrammatic enlarged cross-sectional view illustrating the association of elements during one stage of the performance of a difliusion transfer process, for the production of positive silver prints, the thickness of the various materials being exaggerated; and

FIG. 2 is a view similar to that of FIGURE 1 illustrating the association of elements during one stage of the performance of another diffusion transfer process, for the production of positive silver prints.

It has now been found that silver halide developing agents of the formula:

I NH:

wherein each of R and R may -be the same or different and comprises a methyl or ethyl group, may be employed in diffusion transfer processes, in accordance with the parameters stated hereinafter, to provide high speed silver prints of exceptional quality, either alone or, preferably, in combination with other silver halide developing agents, particularly hydroquinone or one of its derivatives.

One method of preparing the compounds'o'f the generic formula wherein one of R and R is a methyl group comprises reacting 4-nitro-6-(methy l or ethy-D-phenol with a secondary amine and formaldehyde, and effecting reduction of the resultant product.

Alternatively, sulfanilic acid may be coupled into the appropriate .phenol and the resultant bis-alkyl-substituted 4-sulfophenylazophenol reduced with sodium hydrosulfite, according to the procedure of Iaco bs, Heidelberger and Rolf, J.A.C.S. 41, 467 (1919). Where desired, 2,5-dichloroaniline may be coupled into the appropriate phenol, the resultant product reduced catalytically, and the desired product separated by the addition of one equivalent of hydrochloric acid followed by ether.

It will Ibe recognized that the amino group may be substituted, for example, by a lower allcyl group such as a methyl group and that the developing agents of the present invention may be employed in the form of their acid addition salts, such as the hydrochloride, where desired.

In particular, the silver halide developing agent of the present invention is highly useful in so-called high speed diffusion transfer processes wherein high quality silver trans-fer prints are obtained from a silver halide emulsion which has been substantially underexposed in relation to its A.S.A. rating. Such an exposure provides a latent image having a density gradient contfined to the low exposure or toe region of the negative materials characteristic curve. a

The speed of a photosensitive material comprises generally an empirically derived relative measurement which may be defined as a value representing the reciprocal of the exposure required to produce a given result. Any precise definition of speed, therefore, is based upon the selection of a particular result as the standard reference point. A precise quantitative measure of speed has been developed from the work of L. A. Jones et al., as reported in Mees, The Theory of the Photographic Process, the Macmillan Company, New York, 1944, chapters XIX and This Work suggested a system in which negative sensitive materials are assigned a speed that is in terms of the exposure required to give a negative image from which a positive print of specified quality can be produced.

Based on this work, The American Standards Association, Incorporated, has established standards for rating sensitive materials for speed. Under such standards, emulsion speed is considered as a value inversely proportional to the minimum exposure which must be incident upon the negative material, from the scene element of minimum brightness in which detail is visible, in order that a print of excellent quality can be made from the resultant negative. These standards specifytechniques for plotting the characteristic H and D curve of a negative material, that is, the curve relating the logarithm of the original exposure of the negative to density in said negative. The value of speed derived from the standard characteristic curve so determined is specified as equal to the reciprocal of the exposure, E, on the characteristic curve at which the slope is 0.3 time the average slope for a long exposure range of 1.5 of which E is the minimum exposure. The precise method for determining speed in this manner is described in detail in the publication of The American Standards Association, Incorporated, IPH 25- 1954, and titled American Standard Method for Determining Photographic Speed and Exposure Index.

The A.S.A. speed rating is to be distinguished from what is termed the A.S.A. Exposure Index, for use with exposure meters and calculators. The A.S.A. Exposure Index is determined by the formula:

A.S.A. Exposure 'Index=A.S.A. speed/ 4 The exposure index so obtained indicates generally the correct exposure rating to which an A.S.A. calibrated exposure meter must be set in order that it give correct exposure data for producing pictures of satisfactory high quality.

The A.S.A. speed rating is to be further distinguished from what may be termed Diffusion Transfer Process Exposure index. in such processes, the exposure index may be based on a curve relating original exposure of the negative to the density in the resultant positive. It has been found experimentally that the Diffusion Transfer Exposure Index of a silver transfer process may be determined by plotting a characteristic curve of the reflection density of the positive as a function of the log.

exposure of .the negative, determining the exposure in meter-candle-seconds (m.c.s.) at the point on this curve corresponding to a density of 0.50, and dividing the constant, 4.0, by the exposure so determined. The exposure index so obtained indicates generally the correct exposure Transfer Exposure Index can be judged or rated according to sensitometric criteria with exposure, that is, the lumi nance flux reaching a unit area of the photosensitive sur- :face, being measured in m.c.s. The equivalence of the foregoing methods of determining rated A.S.A. Exposure Iudices and Diffusion Transfer Exposure Indices, can be readily established by obtaining a standardized silver transfer positive and determining the illumination, by measuring with an exposure meter calibrated according to an A.S.A. standard which includes a computer in which has [been entered the shutter duration and f-stop of the camera, found necessary to provide the standard transfer print.

In the subsequent discussion, the term A.S.A. Exposure Index is intended to signify the exposure index determined in accordance with the aforementioned American Standards Association specifications. The term Diffusion Transfer Exposure Index is intended to signify, in reference to diffusion transfer processes, or the materials used therein, the exposure index as determined in the aforementioned manner. Both designations, in one sense, serve the same purpose. The A.S.A. Exposure Index of the negative is based upon the exposure to which the negative must :be subjected in order to obtain a good photograph of a predetermined subjectby conventional pnocessing, whereas the Diffusion Transfer Exposure Index is based upon the exposure to which a negative for use in a silver diffusion transfer process must be subjected in order to obtain a good positive by that process.

rating of a silver transfer process to which an exposure Both the A.S.A. Exposure Index and the Diffusion Both, therefore, are direct guides to the exposure setting which must be made in a camera in order to obtain proper exposure. Nevertheless, the two definitions of exposure must be carefully distinguished from each other because, although the A.S.A. Exposure Index is related quali tatively to the production of a positive of high quality, it is a term which describes the character of a negative material. This negative material, nevertheless, may be employed, for example, in a silver diffusion transfer process of the present invention to effect a Diffusion Transfer Exposure tlndex vastly different from the A.S.A. Exposure Index of the negative material. In the novel diffusion transfer processes of the present invention, a positive print of satisfactory high quality is produced from a negative material subjected to exposures less, in some cases many times less, than recommended by its rated A.S.A. Exposure Index. Such an exposure provides a latent image having a density gradient confined to the low exposure or toe region of the negative materials characteristic curve.

Referring now to the drawing, FIGURE 1 illustrates one assemblage and process of the present invention in the performance of a diffusion transfer process for the production of positive silver prints. As depicted in the drawing, an aqueous alkaline fluid layer 12 of a silver halide developing agent chosen in accordance with'the present invention and a suitable silver halide solventare spread between photosensitive emulsion layer 11 which is superposed on support 10 and image-receiving layer 13 which is affixed to support layer 14. Image-receiving layer 13 preferably contains silver precipitating agentsor nuclei. such as the silver precipitating nuclei disclosed in US. Patent No. 2,698,237. Support layer 14 may com- .prise an opaque material where a reflection print is desired or may comprise a transparent material where a transparency is desired.

Fluid layer 12 maybe obtained by distribution of the processing composition in a substantially uniform.man ner between photosensitiveemulsion layer 11 and imagereceiving layer 13, for example, in-accordance with the procedures disclosed in US. PatentNo. 2,543,181. For example, one or more rupturable containers may be attached to either photosensitive emulsion layer 11 and/or image-receiving layer 13 such that upon superposition of the respective layers 11 and 13 said container or containers are so positioned as to be capable, upon rupture, of releasing their contents in a substantially uniform layer between and in contact with the opposed surface of each of said layers. Rupture of the container or containers and spreading of the contents thereof may be accomplished, for example, by compression between a pair of opposed, suitably gapped, rollers.

For clarity, the developing agent, the degree to which the photosensitive layer is exposed, and the character of the silver-receptive layer will be described in detail hereinafter.

The processing composition preferably comprises a filmforming transfer processing composition. It may comprise, for example, one or more of the developing agents in accordance with the present invention, an alkali such as sodium hydroxide, a silver halide complexing agent such as sodium thiosulfate, and a high molecular weight film-forming thickening agent such as sodium carboxymethyl cellulose. All these materials are preferably in aqueous solution. These photographic agents are preferably contained in solution in the processing composition prior to the spreading thereof as layer 12, but they may be in part or in whole added to the processing composition as it is spread between the photo-sensitive emulsion 11 and image-receiving layer 13, said agents being so located on, in or adjacent to a surface of one or both of said layers as to be dissolved by or otherwise interacted with the liquid agent when the latter wets said. layers.

In carrying out the aforementioned transfer process, the photosensitive emulsion 11 is exposed to a predetermined subject matter to form therein. a latent image of said subject matter. The exposed emulsion is superposed on image-receiving layer 13 and the photographic processing composition 12 spreads between the opposed surfaces of said emulsion 11 and said image-receiving layer 13. Reagents permeate into the photosensative emulsion 11, developing the latent image contained therein and forming a soluble silver complex of unexposed silver halide. Soluble silver complex is transported from photosensitive emulsion layer 11, at least in part, by imbibition, to printreceiving stratum 13 and the silver of the complex is precipitated thereon and/ or therein to provide the desired positive image formation. The laminate formed by the sperading of the processing composition as layer 12 between photosensitive emulsion layer 11 and print-receiving layer 13 is kept intact for approximately seconds to 1% minutes, preferably 10 seconds, and at the termination of this time interval the print-receiving layer 13 is dissociated from photosensitive emulsion 11 as, for example, by manual stripping.

A further transfer process of the present invention for the production of positive silver prints is illustrated in FIG. 2 and comprises a spreader sheet 15, a layer of relatively viscous processing composition 12, a photosensitive emulsion layer 11 superposed on image-receiving layer 13 which is, in turn, superposed on a support layer 14. As stated in connection with the description of FIG. 1, imagereceiving layer 13 preferably contains silver precipitating nuclei and support layer 14 may comprise either an opaque or transparent material.

Fluid composition layer 12 may be obtained by spreading a photographic processing composition, for example, in a manner disclosed in U.S. Patent No. 2,698,244. As disclosed in the aforementioned patent, the liquid processing composition may be disposed in a rupturable container so positioned in regard to the appropriate surface of photosensitive emulsion layer 11 that, upon compression by spreader sheet 15, a substantially uniform layer 12 of processing composition is distributed over the external surface of said photosensitive emulsion 11, with respect to image-receiving layer 13.

In carrying out the last-mentioned transfer process, the photosensitive emulsion 11 is exposed to a predetermined subject matter to form therein a latent image of said subject matter. A substantially uniform distribution of processing composition 12 is distributed on the external "surface of said emulsion 11, as for example, according to the previously described procedure. Processing composition reagents permeate into photosensitive emulsion 11, developing the latent image contained therein according to the point-to-point degree of exposure thereof. Substantially contemporaneous with the development of .the latent image, an imagewise distribution of soluble silver complex is formed from unexposed and undeveloped silver halide within said emulsion. At least part of said silver complex, solubilized, is transferred, by imbibition, to print-receiving stratum 13. The transferred silver complexes are reacted therein to provide a positive, reversed image of the latent image. Subsequent to formation of the positive image in image-receiving layer 13, dissociation of said layer from emulsion layer 11 may be effected.

Where desired, the image-receiving layer 13 may be dissociated from emulsion layer 11 by stripping the emulsion from the surface thereof. A conventional stripping layer may be provided to facilitate separation of emulsion layer 11 from image-receiving layer 13 subsequent to transfer processing. Sufiicient abrasion-resistant properties may be provided to image-receiving layer 13 as to alleviate any necessity of subsequently overcoating the external surface of said image-receiving layer 13 with a transparent abrasion-resistant water-soluble plastic to prevent subsequent laceration and resultant degradation of the positive image. Image-receiving layer 13 may also comprise sufiicient integral dimensional stability as to alleviate the necessity of a separate support layer 14.

In the last mentioned processes, spreading of the liquid processing composition on the external surface of photosensitive emulsion layer 11 is preferably effected by rupture of a suitably positioned frangible container and distribution of its processing composition contents by means of a converted cellulose acetate spreader sheet, that is, a cellulose acetate sheet the surface of which has been converted to cellulose; When employed, the converted cellulose acetate spreader sheet may exhibit an adhesive capacity for the processing composition in excess of the adhesive capacity exhibited by the photosensitive emulsion. A means is thus provided for effecting dissociation of the processing composition from contact with the photosensitive emulsion, subsequent to image formation, by dissociating the spreader sheet from its proximate relationship to the external emulsion surface.

It will be apparent that the facility with which the photosensitive emulsion layer is dissociated from contact with the print-receiving layer may be increased by providing a conventional stripping layer interposed between said emulsion and said print-receiving layer. The stripping layer may be coated on the surface of the print-receiving element and a photosensitive emulsion thereafter coated on the external surface of said stripping layer.

While distribution of the processing composition in diffusion transfer processes had been described utilizing a frangible container, it will be apparent that said container provides a convenient means of distributing the liquid processing composition to permit the processing to be effected within a suitable camera apparatus. The diffusion transfer processes of this invention may be otherwise effected. For example, a photosensitive element, after exposing in a suitable apparatus and while preventing further exposure thereafter to actinic radiation, may be removed from such apparatus and permeated with the liquid processing composition as, for example, by coating, spraying, flowing etc., the composition on said photosensitive element or otherwise wetting said element with a composition, following which the permeated, exposed, photosensitive element, still without additional exposure to actinic radiation, is brought into contact with the imagereceiving element for image formation in the manner heretofore described.

The rupturable containers may be constructed in accordancewith the disclosures set forth in U.S. Patent No. 2,638,886. Containers of this type are generally constructed from a blank comprising a flexible, deformable,

three-ply sheet material comprising, respectively, an outer layer of kraft paper, a layer of metal foil and an inner layer or liner of a thermoplastic resin. The container blank is folded upon itself'such as to provide a fluid-containing cavity and a container exhibiting a sealed passage adjacent to an edge thereof which may be substantially uniformly unsealed throughout a predetermined length of the seal passage upon application of stress to the container. The passage may be formed by the utilization of differential adhesion.

As previously noted, the print-receiving stratum preferably contains silver precipitating agents or nuclei, whose presence during the transfer process has a desirable effect on the amount and character of the silver precipitated during positive print formation. Examples of such silver precipitating agents are the metallic sulfides and selenides, thiooxalates, and thioacetamides, and colloidal metals disclosed in U.S. Patent No. 2,698,237. It is also desirable, as disclosed in that patent, to provide, as the vehicle for the silver precipitating agents, a macroscopically continuous film that consists of submacroscopic agglomerates of minute particles of a suitable water-insoluble, inorganic, preferably siliceous, material such as silica aerogel. The use of such a vehicle for the precipitating agents tends to aggregate the silver that is precipitated into its most effective condition for print formation.

Silver halide solvents suitable for incorporation in the processing composition include conventional fixing agents such as sodium thiosulfate, sodium thiocyanate, ammonium thiosulfate, or associations of cyclic imides and nitrogenous bases such as associations of barbiturates or uracils and ammonia or amines. f these, the conventional fixing agents specified are preferred. Preferably, the solution also containers a film-forming material such as a water-soluble plastic, starch or gum imparting a viscosity of from 1000 to 200,000 centipoises at a temperature of 20 C. in order to permit the solution to be readily controlled during and after spreading.

It will be apparent that the relative proportions of the agents of the developer composition set forth herein may be altered to suit the requirements of the operator. Thus, it is within the scope of this invention to modify the herein described developing compositions by the substitution of preservatives, alkalies, silver halide solvents, etc., other than those specifically mentioned. When desirable, it is also contemplated to include, in the developing composition, components such as restrainers, accelerators, etc. Similarly, the concentration of developing agent may be varied over a wide range and when desirable the developing agent may be disposed in the photosensitive element prior to the exposure of the emulsion. The developing agent may be disposed in a separate permeable layer of the photosensitive element and/or in the photosensitive emulsion.

The pH of the developing agent containing solution should be in excess of about 12, to provide the results detailed. The concentration of developer employed ranges from about 1 to 8% and that of the solventfrom 0.3 to 20% by weight.

The emulsion support layer designated in the drawing as 10 may comprise any of the various types of conventional rigid or flexible supports, for example, glass, paper, metal, and polymeric films of both the synthetic types and those derived from naturally occurring products.

The photosensitive emulsion stratum may comprise a commercially available silver halide gelatin emulsion such as sold by Eastman Kodak Company under the trade names Microfile, Spectrum Analysis, Contrast Process, SXX Aero Recon, Verichrome, Royal Pan, Royal X Pan, or Tri X Pan, or sold by E. I. du Pont under the trade names Fine Grain Pan, High Speed Pan, Arrow Pan, or Superior 3, or sold by Ansco under the trade name Triple S. Pan, or sold by Gevaert under the trade name Gevapan.

As previously stated, the silver halide developing agent Example I A photosensitive silver iodobromide emulsion mounted on a paper base was exposed to a predetermined subject through a stop which gives rise to the specified Diffusion Transfer Exposure Index, advanced in superposed relationship with an image-receiving element (both the photosensitive and image-receiving elements comprise commercially available components sold by Polaroid Corporation, Cambridge, Massachusetts, under the trade designation of Type 47), between a pair of pressure-applying rollers to spread a processing composition comprising:

Water ..cc 8,289.0 Hydroxyethyl cellulose (high viscosity) grams 366.0 Sodium sulfite do 260.0 Sodium hydroxide do 425.0 Sodium thiosulfate do 741.0

6-nitrobenzimidazole do 10.8 2,6-dimethyl-p-aminophenol do 270.0 Potassium iodide do 15.0

between the photoexposed emulsion and the image-receiving element in a thin layer approximately 0.003 of an inch thick. After animbibition period of 10 seconds, the emulsion, together with the layer of processing composition, was stripped from the image-receiving element to uncover a positive print, which exhibited the following characteristics:

Diffusionv Maximum Minimum Shoulder Transfer Density Density Speed (n1.c.s.) Exposure Index Example 2 The process of Example 1 was repeated except that the 2,6-dimethyl-p-aminophenol was replaced with 2-methyl- 6-ethyl-p-aminophenol. The positive transfer image exhibited the following characteristics:

Diffusion Maximum Minimum Shoulder Transfer Density Density Speed (m.c.s.) Exposure Index Example 3 The process of Example 1 was repeated except that the 2,6-dimethyl-p-aminopheno1 was replaced with 2,6-diethyl'p-aminophenol. The positive transfer image exhibited the following characteristics:

As used in the above examples, the shoulder speed is the minimum amount of exposure in meter-candleseconds which can be used to expose the negative emulsion and give a transfer image in which there is the minimum contrast which the eye can distinguish, i.e., there is visible detail in the shadow regions of the transfer image. This exposure level is determined at the point on the shoulder portion of the previously-described characteristic curve of the positive transfer image where the slope is 0.4. a The chief function of a photographic negative material as used in pictorial photography is to reproduce as density differences the luminance diiferences existing in the object photographed. The minimum useful exposure will, therefore, be that required to reproduce the minimum difference existing in the shadow regions of the object by means of some minimum density difference in the resulting image.

When the developing agent designated is used in high speed diffusion transfer processes, the region of the ASA. characteristic curve of the negative used in accordance with the present invention lies at a relatively low exposure level and corresponds to an exposure gradient predominantly below approximately 0.015 m.c.s. Ordinarily, the fog level in such a case is at a point on the characteristic curve that corresponds to an exposure gradient predominantly above approximately 0.005 m.c.s.

It has been determined that, under the proper conditions, the activity of the silver halide developing agent, in accordance with the present invention, is concentrated in the extreme outer layer of the silver halide stratum. This phenomenon is probably due to the fact that the activity of the developer is so rapid that oxidation of the developer occurs before it is able to penetrate very deeply into the silver halide stratum. In comparison, the silver halide developing agent of conventional silver diffusion transfer processes is active in deeper layers of the silver halide stratum.

As illustrated in the aforementioned specific examples the photosensitive emulsions of the present processes are underexposed in relation to their respective A.S.A. Exposure Index to produce therein a weak latent image having an exposure gradient predominantly in the toe region of the ASA. density versus log exposure curve of the photosensitive emulsion. The mulsion is therefore exposed to produce a latent image lying in the toe region of the ASA. characteristic curve for the negative emulsion such that, by the standard A.S.A. development method, it would be impractical to develop the emulsion such as to provide a useful conventional negative image. By means of the processes of the present invention, one may advantageously employ a photosensitive emulsion having a stated A.S.A. Exposure Index and provide said emulsion with an effective exposure that ordinarily would require a photosensitive emulsion having a higher A.S.A. Exposure Index to provide an acceptable positive print resultant therefrom.

With conventional developing practices, good negatives can be obtained only from an emulsion that has been sufficiently exposed within the range of exposures specified for the emulsion. Underexposed emulsions result in a negative having only an image from which it is impractical to obtain a satisfactory print by conventional processes.

The present invention obviates the problem of constructing a satisfactory negative from an underexposed emulsion, for example, by intensification, hypersensitization, latensification techniques, so as to obtain a satisfactory positive print. In addition, the present invention provides satisfactory positive images from both underexposed and adequately exposed conventional silver halide photosensitive emulsions. As previously mentioned, a convenient measure of the amplification of image development, obtained according to the present disclosure, over the results obtained from conventional development practices, is in terms of increase in the speed of quantum excitation sensitivity of the emulsion as indicated by the results previously set forth, over the normal specified speed of the same emulsion as determined by the results of standard de velopment practice.

It will be recognized that the steps of the instant processes may, where desired, be combined with such conventional speed-increasing steps as the aforementioned hypersensitization, latensification, etc. In particular, prefogging has been used to excellent advantage.

The advantages detailed herein, however, do not depend upon the presence of, or polymerization of, a polymerizable vinyl monomer and, accordingly, the polymerization of such polymerizable vinyl monomer, as a necessary element of the described process, is hereby expressly excluded.

Since certain changes may be made in the above processes without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A photographic process which comprises the steps of underexposing a photosensitive silver halide emulsion with respect to its A.S.A. Exposure Index to produce therein a latent image having an exposure gradient in the toe region of the ASA. density versus log exposure curve of said silver halide emulsion, developing said emulsion with an aqueous alkaline solution having a pH in excess of about 12 and containing a silver halide developing agent of the formula:

wherein R and R are each an alkyl group selected from the group consisting of methyl and ethyl groups, in a concentration of about 1 to 8%, by weight; contacting unreduced and undeveloped silver halide therein with a silver halide solvent to form an imagewise distribution of soluble silver complex; transferring at least part of said imagewise distribution, by imbibition, to a contiguous image-receiving material containing silver precipitating agents; and thereby effecting precipitation of transferred soluble silver complex to provide a positive silver print of full pictorial density on said image-receiving material.

2. A photographic process as defined in claim 1 wherein said agent is 2,6-dimethyl-p-aminophenol.

3. A photographic process as defined in claim 1 wherein said agent is Z-methyl-6-ethyl-p-aminophenol.

4. A photographic process as defined in claim 1 Wherein said agent is 2,6-diethyl-p-aminophenol.

5. A photographic process as defined in claim 1 wherein said aqueous alkaline solution includes a hydroquinone silver halide developing agent.

6. The photographic process which comprises the steps of selectively exposing a photosensitive gelatino silver halide emulsion with a luminance flux incident thereon not substantially in excess of 0.015 meter-candle-seconds; developing exposed silver halide in said photosensitive emulsion with an aqueous alkaline solution having a pH in excess of about 12 and containing a silver halide develop ing agent of the formula;

wherein R and R are each an alkyl group selected from the groupconsisting of methyl and ethyl groups, in a concentration of about 1 to 8%, by weight, and a silver halide solvent in a concentration of about 0.3 to 20%, by weight; contacting unexposed and undeveloped silver halide therein with said silver halide solvent and forming thereby an imagewise distribution of a soluble silver complex in the unexposed areas of said emulsion, as a function of the point-to-point degree of exposure thereof; transferring from said emulsion, at least in part, by imbibition, said imagewise distribution of soluble silver complex to a print-receiving layer, containing silver precipitating nuclei, in superposed relationship to said emulsion; and there precipitating silver complex to provide thereby a reversed positive, full scale silver print of the latent image.

7. The photographic process which comprises the steps of exposing a photosensitive gelatino silver halide emulsion with a light flux incident thereon predominantly within the exposure range delineated by the toe region of said emulsions characteristic H and D curve, determined according to A.S.A. Standard PH 2.5-1954; developing exposed silver halide in said photosensitive emulsion with an aqueous alkaline solution having a pH in excess of about 12 and a silver halide developing agent of the formula:

IIQ'H) wherein R and R are each an alkyl group selectedfrom the group consisting of methyl and ethyl groups, in a con-'. centration of about 1 to 8%, by weight; contacting undeveloped silver halide with a silver halide solvent and forming thereby an irnagewise distribution of soluble silver complex in the unexposed areas of said emulsion; transferring from said emulsion, at least in part, said imagewise distribution of soluble silver complex to a printreceiving element, containing silver precipitating agents, in superposed relationship to said emulsion; and precipitating said silver complex to provide thereby a reversed, positive print possessing a characteristic curve with a slope greater than the slope of said H and D curve, said positive prints characteristic curve determined by plotting the reflection density of the positive print as a function of the log exposure of the negative.

References Cited by the Examiner UNITED STATES PATENTS NORMAN G. TORCHIN, Primary Examiner.

J. T. BROWN, Assistant Examiner. 

1. A PHOTOGRAPHIC PROCESS WHICH COMPRISES THE STEPS OF UNDEREXPOSING A PHOTENSISITIVE SILVER HALIDE EMULSION WITH RESPECT TO ITS A.S.A. EXPOSURE INDEX TO PRODUCE THEREIN A LATENT IMAGE HAVING AN EXPOSURE GRADIENT IN THE TOE REGION OF THE A.S.A. DENSITY VERSUS LOG EXPOSURE CURVE OF SAID SILVER HALIDE EMULSION, DEVELOPING SAID EMULSION WITH AN AQUEOUS ALKALINE SOLUTION HAVING A PH IN EXCESS OF ABOUT 12 AND CONTAINING A SILVER HALIDE DEVELOPING AGENT OF THE FORMULA: 